The present invention relates to a combined ball and-damper device for mounting between a flexible spar connecting a helicopter blade to a rotor hub, and a torque tube for controlling the pitch of said blade.
U.S. Pat. No. 4,244,677 and EP patent No. 0 451 084 (United Technologies Corporation) disclose combined devices each comprising two individual modules, each module comprising a laminated damper and a laminated ball. The laminated damper comprises a stack of alternating layers of metal and of elastomer, extending between an outer end an inner end of the damper. At its outer end, the laminated damper has an assembly element connected to the pitch control torque tube, and at its inner end, it is secured to an outer end of the ball of the module, whose own inner end presents an element for fixing to the flexible spar.
Such a combined device (also referred to as a xe2x80x9csnubberxe2x80x9d) is used with a flexible spar of a rotor hub which does not present any ball bearings (known as a xe2x80x9cbearingless main rotorxe2x80x9d or BMR), and it serves to position the components of the rotor (blades/flexible spar/torque tube for controlling pitch) and to provide damping of the natural mode of vibration of the blades.
A BMR type rotor hub is constituted by a flexible spar structure, i.e. a strip, generally of composite material, connecting the blade to the rotor hub and tensioned by the centrifugal force imposed by the rotor rotating about said hub. A torque tube surrounds the spar, being fixed to the base of the blade and serving to impart twisting about the main axis common to the blade and to the spar. This xe2x80x9cpitchxe2x80x9d control enables the helicopter to be controlled.
Given the forces and the displacements that result from this imposed pitch control and the aerodynamic interaction between the blade and the surrounding air, and also the interaction between the rotor assembly and the structure of the helicopter, it is necessary:
to maintain the positioning (centering) of the torque tube relative to the spar. This is an abutment or xe2x80x9csnubberxe2x80x9d function serving essentially to take up the compression forces due to the blade flapping;
to decouple the torque tube and the spar in terms of rotation about their common axis. The twisting imposed by controlling pitch gives rise to highly differing amounts of twist between the torque tube and the spar due to the geometry of the system. The combined device must therefore accommodate this differential twisting; and
to accommodate displacement in translation due to the aerodynamic drag on the blade associated with its rotation. Drag imparts shear that alternates in direction in the general plane of the rotor, perpendicularly to the above-described plane of excitation in flapping.
Each of the two above-mentioned documents (U.S. Pat. No. 4,244,677 and EP 0 451 084) describes a combined device (a xe2x80x9csnubberxe2x80x9d) itself constituted by two modules, each comprising a stack made up of a generally spherical portion and a generally plane portion, each portion being made of different materials, and together providing the ability to satisfy the three above-mentioned functions.
The laminated structure provides the ability to withstand compression or xe2x80x9cflappingxe2x80x9d forces. The generally spherical shape also makes it possible to provide the system with rotary decoupling. The plane portion serves to accommodate the movement in translation associated with aerodynamic drag.
Those combined ball-and-damper modules are associated opposite ways round in pairs to form respective assemblies for use in installing and operating each of the blades of the rotor.
U.S. Pat. No. 4,244,677 essentially describes means for externally applying prestress. That prestress is applied simultaneously to both of the two modules each comprising a plane damper and a ball in order to guarantee that there is no traction stress in the structure of either element, thereby extending its lifetime. It should nevertheless be observed that that prestress is the same both in the plane damper and in the ball.
The Applicant company has found that a combined ball-and-damper structure as described above leads to two fundamental problems which prevent optimum functioning of the rotor and of the combined snubbers themselves:
the rotary decoupling provided by the spherical portion of the combined unit is incomplete because of the general flexibility of the damper units; and
the combination of loads and displacements that are imposed can be computed only by taking account of the mutual interaction of those loads and displacements, and of their interactions with the particular flexibility of the combined snubber elements about their various axes,
which means that the instantaneous center of rotation of the torque tube depends on the combination of imposed displacements and loads and lies neither on the geometrical center of the installed combined snubber unit, nor on the main axis of symmetry of the sleeve; and
the combination of loads that result from the flexibilities is complex, and it depends on the particular embodiment of the structures of the combined snubber elements.
It results from the above that there exists a need to improve the lifetime of such combined ball-and-damper devices, while remedying at least one of the above-mentioned problems.
The present invention relies in particular on recognizing the fact that in the presence of shear stresses in aerodynamic drag mode, the two intermediate plates interconnecting the spherical portions with the plane portions do not remain parallel.
The idea on which the present invention is based is to introduce a stiffening or securing connection between the two modules via their interfaces between their plane laminated dampers and their balls.
The invention thus provides a combined ball-and-damper device for mounting between firstly a flexible spar connecting a helicopter blade to a rotor hub and secondly a torque tube for controlling the pitch of said blade, the device comprising two individual modules each comprising a plane laminated damper and a laminated ball, whose damping action can be provided in conventional manner by rubber or by an internal hydraulic device, the laminated damper extending between an outer end and an inner end and presenting at its outer end an element for connection to the torque tube for controlling the pitch of said helicopter blade, and the laminated ball presenting a stack of metal and elastomer layers, the laminated damper being secured at its inner end to an outer end of the ball whose own inner end presents an element for fixing to said flexible spar, the device including at least one connection element providing a stiffening connection between the outer ends of the two balls of the two individual modules making up the combined device.
Said connection between the inner end of the plane laminated damper and the outer end of the ball can be provided by means of at least one intermediate plate.
In a first variant, the connection element is a tube surrounding both balls and providing said connection between the laminated dampers and the balls of the two individual modules constituting the combined device.
In a second variant, the connection element is an open tube providing the connection between a laminated damper and a ball in one of the two individual modules, with the open edges of said open tube being secured to the outer end of the ball of the other individual module.
In another variant, the connection element is a piece interconnecting the facing outer ends of the two balls of the combined device. Said piece is advantageously curvilinear and presents a concave side facing towards the balls of the combined device, thus making it possible to go round the side of the flexible spar.
In another variant, a said piece is made up of tie bars, there being four tie bars, for example.
In a preferred variant, the device has a pair of connection elements each presenting a central region forming a connection between the inner end of a laminated damper and the outer end of a ball, and lying on at least one side beside a side region secured to a corresponding side region of the other connection element of the pair.
The connection element(s) may advantageously be dimensioned in such a manner that after the combined device has been assembled between a flexible spar and a pitch control tube, a first amount of compression prestress is applied to the two balls of the combined device.
The elements for assembling the combined device on the pitch control tube are advantageously dimensioned in such a manner that once the combined device has been assembled, a second amount of compression prestress is applied to the laminated dampers.
The first and second amounts of prestress may have different values, which makes it possible to select each of them in optimum manner for each of the elements (the damper and the ball) of the combined device.
The invention also provides a helicopter rotor comprising at least one combined device as defined above mounted between a flexible spar and a pitch control tube of a helicopter blade. Such an assembly can be provided in such a manner as to apply a first amount of compression prestress to the balls, and/or a second amount of compression prestress to the laminated dampers, with the first and second amounts of prestress advantageously being different.